Let's talk some more about shading, in fact, let's
talk a LOT more about shading. If your goal is to make
the drawing look as real as possible, then shading is
going to go the furthest to make that happen. Real life
objects are seen in three dimensions - objects have
height and width and depth, and what most makes the
object look 3-D is the third dimension - depth. BUT.....
if you look at it logically and think about your paper
that you'll be drawing on, the paper has height (it has
a top and bottom) and it has width (from side to side),
but it has almost no depth. If you look at a standard
piece of printer paper (it used to be called typing
paper) the paper is 11 inches tall (height) and 8 and a
half inches wide (width), but it's depth (look at it
from the side) is almost nothing. So what you're really
working with is a two dimensional drawing surface. I
know you know that about paper, that it's "paper thin"
(ha ha), I'm just trying to show that paper,
practically, has only two dimensions. But
if what makes drawing look 3-D is the depth, how are you
going to create depth when you can't draw in the third
dimension. The answer is SHADING!

Look at these two cubes. The one on the left looks
like a cube all right, but it's very flat, and not very
realistic looking. If, for example, you looked at a six
sided die, you wouldn't see the image on the left -
thick lines outlining the shape of the die, and the
light appears exactly the same from all angles of the
die. It's more likely that you'd see the image on the
right. This is the same drawing as the one on the left,
but the one on the right has shading added to it. And
the result is the image on the right looks more
realistic, this is the way your eye might actually see a
die (without the black dots). The difference between the
two images is one has shading, the other doesn't.